Conventionally, exhaust gas emitted from combustion systems such as waste incinerators, boilers and the like is diffused into the atmosphere after being purified by a gas purification device.
In some cases of purification treatment, it has been found necessary that the temperature of the exhaust gas be reduced to an appropriate temperature, for example, approximately 120-250° C. depending on the gas purification device used. Conventionally, in such cases, gas purification devices spray water into the exhaust gas and utilise its heat capacity and latent heat of evaporation to reduce the temperature of the exhaust gas.
Referring to FIG. 9 and FIG. 10, examples are shown of a conventional device to reduce the temperature of exhaust gas, wherein 21 is a gas cooling chamber, 21a is an exhaust gas inlet, 21b is an exhaust gas outlet, 21c is an ash outlet, 22 is a temperature reduction water tank, 23 is a pressure pump, 24 is a temperature reduction water nozzle, 25 is a temperature control device, 25a is a temperature detector, 26 is a temperature reduction water volume control valve, 27 is a injection pump, 28 is an air compressor, 29 is a compressed air tank, 30 is a mixer, Gh is high temperature exhaust gas, G1 is low temperature exhaust gas and C is ash.
With reference to the device for reducing the temperature of exhaust gas in FIG. 9, high pressure water from the temperature reduction water tank 22, pressurized by the pressure pump 23, is sprayed into the gas cooling chamber 21 through the temperature reduction water nozzle 24 provided in the vicinity of the exhaust gas inlet 21a. The temperature of sprayed water rises in contact with high temperature exhaust gas Gh, and is vaporised to become steam when it reaches its boiling point.
On the other h and, high temperature exhaust gas Gh in the gas cooling chamber 21 is cooled by the heat capacity of the sprayed water, latent heat of evaporation and the heat capacity of the steam, thus lowering the temperature to a prescribed temperature so as to be led out of the exhaust gas outlet.
The volume of water to be sprayed into the gas cooling chamber 21 is controlled by adjusting the opening of the temperature reduction water volume control valve 26 through the temperature control device 25 in response to temperature detecting signals from the temperature detector 25a. The temperature of low temperature exhaust gas G led out of the exhaust gas outlet 21b is maintained at a desired temperature by controlling water volume to be sprayed into the gas cooling chamber 21 by means of controlling the volume of water returned to the temperature reduction water tank 22.
With reference to the device for reducing the temperature of exhaust gas shown in FIG. 10, water sent from the temperature reduction water tank 22 by means of the injection pump 27 and high pressured air sent from the compressed air tank 29 are mixed for atomisation in the mixer 30. Atomised water is sprayed into the gas cooling chamber 21 from the mixer 30, through the temperature reduction water nozzle provided in the vicinity of the exhaust gas inlet 21a. 
Features such as (1) that the temperature of sprayed water rises in contact with high pressure exhaust gas Gh and is vaporised to become vapour steam when it has reached its boiling point, (2) that high temperature exhaust gas Gh in the gas cooling chamber 21 is cooled by the heat capacity of the sprayed water, latent heat of evaporation and heat capacity of the steam vapour, (3) that the water volume to be sprayed into the gas cooling chamber 21 is controlled by adjusting the opening of the temperature reduction water volume control valve 26 through the temperature control device 25, and (4) that the temperature of low temperature exhaust gas G is maintained at a desired temperature by controlling water volume to be sprayed, are all precisely the same as those features in FIG. 9. The previous devices for the temperature reduction of exhaust gas shown in FIGS. 9 and 10 are capable of reducing the temperature of high temperature exhaust gas Gh to a desired temperature by utilising low cost water, thus achieving excellent and practical effects.
There remain, however, a number of difficulties related to the aforementioned prior devices for temperature reduction of exhaust gas, of which, major difficulties include (a) that refractories are damaged by the downflow of water droplets when they hit the wall surface of the gas cooling chaser directly, (b) that stable operation of the gas cooling chamber is impaired by dust adhered to and deposited on the wall surface, and (c) that it is difficult to provide a device for temperature reduction of exhaust gas having a small size since the gas cooling chamber remains large in size.
In the event of a single fluid method wherein only water is utilized, as shown in FIG. 9, difficulties remain in making the atomized temperature reduction water have particles of micro-sized diameters, even by increasing the pressure of the water or making improvements in the water nozzle 24. With this method, the diameters of atomised particles of temperature reduction water normally stay coarse, having diameters around 70-200 μm, which makes it difficult for the atomised temperature reduction water to be thoroughly vaporised within a limited space, thus causing damage to the refractory when water droplets hit the wall surface of the gas cooling chamber directly.
Even when damage to the refractory is avoided, there is a possibility that dust would adhere to and deposit at the surface of the refractory that is wet with water droplets, that deposits adhered to the surface of the refractory would gradually grow, and that the passage resistance of exhaust gas in the gas cooling chamber would increase and fluctuate considerably, thus making the smooth operation of the gas cooling chamber difficult.
With a double fluid method shown in FIG. 10, wherein water and compressed air are employed, the diameters of atomised particles of temperature reduction water normally become around 30-100 μm thus reducing the frequency of problems in comparison with the single fluid method.
However, this double fluid method is not ideal from the viewpoint of cost because of the high initial and running costs of compressed air equipment.
Furthermore, the time required before the aforementioned atomised cooling water reaches its boiling point and evaporates thoroughly is considerably long. This means that it becomes necessary for the retention time of exhaust gas in a gas cooling chamber to be sufficiently long, thus requiring a gas cooling chamber of a large capacity.
For example, in the case of an industrial waste incinerator with a capacity to handle incineration disposal of industrial waste of approximately 300 T/D (tons per day), assuming high temperature exhaust gas Gh with an exhaust gas volume of 90,000 Nm3/H (flow rate of gas with volume converted to normal or standard volume) and an inlet exhaust gas temperature of 240° C. is converted to low temperature exhaust gas G with an inlet gas temperature of 180° C., a gas cooling chamber of an internal diameter of approximately 4,800 mm and the height of approximately 9,000 mm is required with a device for temperature reduction of exhaust gas by means of a single fluid method as shown in FIG. 9. Thus, the total height of the device for temperature reduction of exhaust gas including an exhaust gas inlet 21a, an exhaust gas outlet 21b and an ash outlet 21c would be approximately 180,000 mm.
When designing previous devices for temperature reduction of exhaust gas, the heat load of the gas cooling chamber is normally chosen to have a value of 5,000-10,000 kcal/m3·H (heat value taken away from exhaust gas per unit volume and unit time of a gas cooling chamber in units of kilocalories per meter cubed per hour). For example, the heal load of the gas cooling chamber is chosen to be 7,000 kcal/m3·H.